1. Field of the Invention
The present invention relates to toner for developing an electrostatic image, for use with image forming methods such as electrophotography, electrostatic recording, electrostatic printing, and the like.
2. Related Background Art
There are many known methods for electrophotography, such as those disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publication No. 42-23910, and Japanese Patent Publication No. 43-24748. Generally, a photo-electroconductive material is employed to form a latent electrical image upon a photosensitive member by means of a variety of means, then the aforementioned latent image is developed using toner and transferred as necessary to a transfer mediums such as paper, and the image is fixed to the transfer medium by means of heating, pressurizing, heating and pressurizing, or solvent vapor, thereby obtaining a toner image.
While there are may methods and apparatuses which have been developed relating to the aforementioned final process of fixing the toner image to a sheet such as paper, at the present, the most common method is the pressurizing-heating method using a heating roller. The pressurizing-heating method using a heating roller fixes a toner image to the fixing sheet material by causing the sheet on which the image is to be fixed to pass across a heating roller which has separatability regarding toner, in such a manner that the surface of the roller comes into contact with the toner image side of the fixing sheet and applies pressure thereto. This method allows of speedy fixing, since the thermal efficiency of fusing the toner image upon the fixing sheet is excellent, due to the heating roller and the toner image on the fixing sheet coming into direct contact under pressure.
The present state is, though, that different toners are being used for the various types of photocopiers and printers. This is mainly due to the difference in fixing speed and fixing temperature of the apparatuses. These differences are resultant of the fact that an offset phenomena is greatly affected by the fixing speed and fixing temperature. The offset phenomena is a phenomena wherein part of the toner image, being in a molten state when coming into contact with the surface of the heating roller, adheres to the surface of the fixing roller during pressurization, and the toner which has adhered to the fixing roller is then re-transferred to the next sheet, thereby soiling it. Generally, the heating roller temperature is set at a low temperature in the event that the fixing speed is slow, and the heating roller temperature is set at a high temperature in the event that the fixing speed is fast. This is to stabilize the amount of heat provided to the toner by the heating roller for fixing the image at an approximate constant, regardless of the fixing speed.
However, there are several layers of toner formed on the sheet to which the image is to be fixed. Accordingly, particularly with systems wherein the fixing speed is fast and the heating roller temperature is high, the temperature difference between the toner layer which comes into contact with the heating roller and the toner layer wich is in contact with the fixing sheet becomes extremely great. Consequently, the topmost layer exhibits toner offset if the temperature of the heating roller is high. On the other hand, if the temperature of the heating roller is low, the toner in the bottom-most layer is not molten sufficiently, meaning that the toner is not completely fixed to the fixing sheet, thereby resulting in a phenomena called low-temperature offset.
A method generally practiced in order to solve this problem is to cause toner anchoring to the fixing sheet by raising the contact pressure during fixing, in the event that the fixing speed is fast. By employing such a method, the temperature of the heating roller can be lowered to a certain extent, and high-temperature offset phenomena occurring at the topmost layer can be prevented. However, this causes an extremely great shearing force to be placed on the toner, resulting in wrapping offset wherein the fixing sheet is wrapped onto the fixing roller, or visibly leaving traces of separating members for separating the fixing sheet from the fixing roller, such as separating claws, on the image. Moreover, this method can cause deterioration of the fixed image such as lines being smashed out of form during fixing or toner being splattered on the image, due to the high pressure.
Accordingly, high-speed fixing is generally conducted by using toner which is lower in melting viscosity than that used for fixing at slower speeds, thereby lowering the heating roller temperature and fixing pressure, so that the fixing can be conducted while preventing high-temperature offset and wrapping offset. However, when such toner with a low melting viscosity is employed for low-speed fixing, offset tends to occur at higher temperatures, owing to the low viscosity thereof.
Methods for lowering the viscosity of the toner include such as lowering the glass transition point of the polymers or lowering the molecular weight thereof. However, in the event that the former method is employed, the storability of the toner is diminished, and in the event that the latter method is employed, the ability to avoid offset at high temperatures and frictional electrification properties deteriorate, and further, adhesion of toner to the photosensitive member occurs more easily. Regarding methods for increasing the degree of branching of polymers on vinyl resins, disclosed in Japanese Patent Application Laid-Open No. 3-87753 and Japanese Patent Application Laid-Open No. 3-203746 is a method using macro-monomers, and disclosed in Japanese Patent Application Laid-Open No. 4-24648 is a method using .epsilon.'-caprolactone-modified hydroxy vinyl monomers. However, when much macro-monomers are used in these methods to make higher the branching degree, the glass transition temperature of the resin is lowered, thereby deteriorating the storability.
As for methods for maintaining the storability of the toner at a favorable level, the following may be given: increasing the molecular weight of the main chain of the polymer so as to raise the glass transition temperature thereof; or altering the monomer composition of the main chain of the polymer so as to raise the glass transition temperature without changing the molecular weight. However, whichever of these methods is used, the fixing temperature rises, and consequently the effects of lowering fixing temperature by means of increasing the degree of branching are diminished. This is owing to the great effects of the composition of macro-monomers in the polymer. Polymerization of a great quantity of macro-monomers within the polymer chain worsens the storability of the toner. Accordingly, in order to improve the storability of the polymers, the glass transition temperature of the main chain must be further raised for the sake of some of the polymers which have an uneven distribution of a great quantity of macro-monomers, resulting in deterioration of fixability. In other words, the increase in viscosity due to the increase in glass transition temperature of the main chain cancels out the effects of lowering viscosity by means of branching, owing to the overly-great difference in the glass transition temperature between the main chain and the branched chain.
As for methods for increasing the degree of branching of polyester resins, methods using polycarboxylic acid or polyahydric alcohol, having 3 or more hydroxyl or carboxyl groups, or using dicarboxylic acid with side chain or diol with side chain, are disclosed in Japanese Patent Application Laid-Open No. 59-228658 and Japanese Patent Application Laid-Open No. 62-195678. However, as mentioned regarding vinyl resins, the increase in glass transition temperature of the main chain cancels out the effects of lowering viscosity attempted by means of lowering the glass transition temperature of the polymer and increasing branching, since the side chain of dicarboxylic acid or diol with side chain are of aliphatic groups. Regarding the method using the polycarboxylic acid or polyhydric alcohol, the degree of branching is increased, but the gel content (matter insoluble in THF) increases as well, so that while high-temperature offset is improved, the fixing temperature increases.
In Japanese Patent Application Laid-Open Nos. 63-225244 through 225246, methods using a blend of two types of non-linear polyester with differing softening points are disclosed, and disclosed in Japanese Patent Application Laid-Open No. 5-249736 is a method using a resin comprised of high-density crosslinkage micro-gel particles containing linear portions and crosslinked portions.
With the former method, adjustment of the degree of branching is extremely difficult, since preparation thereof is made with dicarboxylic acid and dihydric alcohol being placed in the same container with polycarboxylic acid or polyhydric alcohol having 3 or more hydroxyl or carboxyl groups.
As can be understood, there is demand for a toner for heating and pressurizing fixing which has a wide fixing temperature range suitably adaptable to speeds both low and high, and has excellent anti-offset properties.
Further, in recent years, digitalization of photocopiers and further reduction in the size of toner particles proceed while aiming at higher image quality in copied images. It is required that photograph images containing characters exhibit clarity in the characters and that photograph images themselves exhibit density gradation true to the original. Generally, when taking copies of photograph images containing characters, raising the line density in order to make the characters clear not only degrades the density gradation of the photograph image, but also causes a very rough image at the half-tone portions.
Further, raising the line density may result in a phenomena called "hollowing-out", wherein the large amount of toner deposited on the recording medium during the toner transfer process causes some of the toner to adhere to the photosensitive member against which it is pressed, thereby pulling out some of the toner in the central portions of lines, hence the term. It is needless to say that an image with hollowing out is of poor quality. On the other hand, attempts to improve the density gradation properties of the photograph image lowers the character line density, thereby reducing clarity.
While miniaturization of the toner particles can improve the resolution and clarity of the image, various problems are apt to occur.
In the first place, miniaturization of toner particles diminishes the fixability of half-tone portions. This phenomena is particularly evident in high-speed fixing. This is because there is little toner deposited on the half-tone portions, and the toner transferred to the concave portions of the fixing sheet receive little heat from the heating roller, in addition to the pressure thereof being controlled by the convex portions of the fixing sheet. The toner particles transferred to half-tone images at the convex portions of the fixing sheet are each subjected to shearing force much greater than that of solid portions where the toner layer is thick. This is because the toner layer is thin, and such a condition may result in offset phenomena and copied images of low quality.
Further, making the toner particles smaller increases the surface area of the toner per unit weight, thereby increasing the range of toner charge distribution, consequently making it easier for fogging to occur. When the surface area of the toner per unit weight is increased, the frictional electrification properties of the toner is liable to be affected by the environment. If the toner particles are made to be too small, the dispersion state of magnetic material and colorant tends to more affect the charging properties of the toner. When such small toner particles are used in high-speed photocopiers, over-charging tends to occur in low-humidity conditions, causing fogging and decrease in density.
In using multi-functional photocopiers which use light exposure to leave part of an image blank for inserting an image of another color in multi-color copying, or to create a blank frame around the edges of the copying paper, fogging tends to occur at the image portions which have been left blank. When strong light is used to erase the image by applying a potential which is inverse to the latent image potential by means of light-emitting diodes or fuse lamps, the tendency for fogging to occur at that portion increases even further.